The present application relates to the field of MEMS micro-sensors or MEMS dynamic sensors, in particular to produce microphones or pressure sensors, in particular of the relative type.
MEMS microphones are increasingly present in general public applications (such as mobile telephones, camcorders, cameras, . . . ).
These components generally use a membrane able to deform as a function of the pressure exerted by the sound to be detected, and a means for detecting that vibration via capacitive means associated with said membrane.
The principle of this sensor is explained in the article by S. Chowdhury et al. “Nonlinear Effects in MEMS Capacitive Microphone Design.” Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS 2003.
FIG. 1A shows the structure of a microphone 1, as explained by Matthias Winter et al. in “Influence of a chip scale package on the frequency response of a MEMS microphone”—Microsystem Technologies, December 2009, DOI 10.1007/s00542-009-0994-z. This publication also provides the equivalent electrical diagram illustrated in FIG. 1B.
A MEMS chip 12 and an ASIC 14 are fastened by connecting rods 18 to a ceramic substrate 2, which has through holes 4, making it possible for a pressure wave to reach the sensor strictly speaking, made up of a perforated counter electrode 6 and a circular membrane 8. The latter part has small openings forming vents to make it possible to offset the static pressure. Behind the membrane is a space 10 closed by a polymer layer and protected by a copper metallization.
The diagram of FIG. 1B show the (acoustic) resistance Rp brought by the presence of the holes 61 into the stationary electrode 6 that the pressure wave must pass through before being able to exert a force on the membrane 8. A second acoustic resistance (Rgap) comes from the damping of the membrane 8 due to the displacement of the air gap 19 between it and the stationary electrode 6. These two resistances therefore need to be minimized to increase the sensor's sensitivity.
The reduction of the resistance Rp is done in particular by increasing the number of holes 61 in the stationary electrode 6. These perforations can reach close to 20% of the total surface of the electrode as explained in the article by A. Dehe “Silicon microphone development and application.”—Sensor and Actuators A 1333:283-287.
These perforations decrease the useful surface of the measuring capacitance proportionately, and therefore the sensitivity. A compromise must therefore be found.
In the aforementioned article, the author provides characteristic dimensions of these microphones.
It is also specified that the bandwidth of the sensor is also highly dependent on the sizing of the perforations 61 and the air gap 19.
The air gap 19 and the perforations 61 also play a decisive role in the noise of these microphones as indicated by M. Brauer et al. “Improved signal-to-noise ratio of Silicon microphones by a high-impedance resistor,” J. Micromech. Microeng. 14 (2004) 86-89.
In all of the known examples, the following problems are seen.
The membrane 8, which serves both as mechanical spring and mobile electrode, recovers the acoustic signal, and is still correlated to the detection electrode 6. It is consequently not possible to optimize the acoustic part of the sensor separately from the electrical measuring part. This is characterized in particular by:                a significant loss of sensitivity due to the presence of a large number of holes in the membrane 6, necessary from an acoustic perspective to reduce the acoustic resistance, but which greatly reduces the opposite surfaces for the capacitive detection; a reduction of the opposite surfaces of up to nearly 20% is also noted,        a viscous damping, determined, among other things, by the air gap 19 between the membrane 8 and the stationary detection electrode 6. The gap itself depends on the pressure range to be measured (maximum bending allowed by the membrane) and the reading voltage (“pull-in” limit).        
All of the known microphone structures are based on the use of a flexible membrane 8 embedded on its periphery. This means that, under the effect of outside pressure, the membrane deforms primarily in the center, but practically not at all on its periphery. As a result, only a fraction of the deformation can be used for capacitive detection.
Furthermore, it is necessary to apply a voltage between the membrane 8 and the reading electrode 6 to read the capacity variation resulting from the deformation of the membrane under the effect of the acoustic pressure to be measured. To limit the measuring noise and increase the sensitivity of said microphone, this voltage must be maximized. However, this maximization is difficult to ensure because it assumes taking several parameters into account, and in particular the size of the air gap and the maximum pressure to be measured (operation near the “pull in” voltage, i.e. the voltage for which the membrane adheres on the control electrode, is sought).
Other problems should be signaled:                the known components are very sensitive to variations in the production method. The sensitivity of the microphone is in fact quite varied as a function of the thickness of the membrane and stresses in the material,        the response is nonlinear as a function of the acoustic pressure, the detection capacity being proportionate to the opposite of the distance between the electrode 6 and membrane 8,        the pressure range and the resistance to pressure shocks are very limited, the structure and the production method making it difficult to place stops in the air gap.        
The problem therefore arises of finding a new sensor or micro-sensor structure of the MEMS type or a dynamic sensor of the MEMS type not having the above drawbacks and limitations.
The problem also arises of finding a new method embodiment for such a structure.